The present invention relates in general to longitudinal cancellation circuits for use with telephone signal systems, and, in particular, to a longitudinal cancellation circuit in which precise matching of current and voltage sources, and other passive components is not required.
Long cancellation circuits are well known in the prior art. These devices can use voltage sources to drive the differential signal onto the line. If the longitudinal cancellation approach is used within this path, it necessitates precisely matched feed resistors to obtain good longitudinal balance. If the feed resistors are not matched, the longitudinal signal converts into a differential signal proportional to the mismatch. It is then impossible to differentiate between the telephone differential signal received, and the longitudinal converted differential signal.
It is also known in the prior art, that this problem of matching feed resistors can be eliminated by the use of voltage-controlled current sources connected to the tip-and-ring terminals of the circuit. If the current sources are identical, the longitudinal balance becomes independent of the feed resistors. However, it has been found that in order to ensure that the current sources are matched, extremely high cost circuit elements are required.
This problem in the prior art is overcome by the present invention which allows the matching of the current sources to be relaxed enough to make the circuit economical to manufacture.